Hidden within Lake Xochimilco in Mexico City, lives the axolotl. With a collar made of gills and a paddle-like tail, this slow, slimy creature spends its days swimming along the lake floor, sucking up insects that come into its path. With its alien body and blank stare, this under water critter has been capturing the hearts of budding biologists and animal enthusiasts for decades. But what actually is an axolotl, and does it warrant all the hype it receives?
The axolotl (scientific name Ambystoma mexicanum) is part of a group of animals known as amphibians who are typically known for their slick, slimy skin and reliance on a moist environment for survival. Axolotls are a type of salamander; one of the more commonly known groups of amphibians (others groups include frogs and toads). What really sets amphibians apart from their reptilian relatives is their process of metamorphosis. In this process, an animal goes through at least two distinct body changes over the course of its life. A classic example would limbless, tailed tadpoles transforming into hopping, adult frogs. However, this is where the axolotl sets itself apart. Not only do axolotls rarely go through metamorphosis, but most remain in an aquatic “larval” form for the duration of their lives (a process called neoteny). Most other salamanders either undergo metamorphosis by losing their gills and becoming partially terrestrial by adulthood (eg. Taricha salamanders) or are born looking like miniature adults (eg. Ensatina salamanders).
Since axolotls resemble salamander larvae, in the early 1800’s researchers believed all axolotls were juveniles, and that their adult form just alluded detection in the wild. It wasn’t until an unexpected axolotl mating in a lab and the subsequent presence of offspring that scientists finally conceded they could, in fact, be adults. However, a small few of these offspring did, strangely, transform into air-breathing adults able to live on land. So what was going on? Well, in 1876 Marie von Chauvin demonstrated that this change could be induced by shifting the axolotls’ environment slowly over time from aquatic to terrestrial. Years later, additional research showed that this metamorphosis could be initiated purely by injecting particular thyroid hormones directly into their body, and that this change could occur at virtually any age of axolotl development (Prahlad & DeLanney 1965). Although other neotenic amphibians exist, this “transformation by injection” has only been demonstrated in axolotls.
So, it appears that in most natural situations, axolotls are able to retain their youthful, juvenile appearance. This ability to “stay young” is also aided by their unique trait to entirely regenerate damaged limbs without scarring. Although many salamanders have this ability, axolotls can go so far as to regenerate jaws, spine, and even brain tissue (Maden et al. 2013). As such, they have been a staple organism for studying the processes that lead to tissue regeneration. Surprisingly, when an axolotl does go through metamorphosis their regenerative powers are significantly reduced, which might explain their apprehension to change form in the first place (Monaghan et al. 2014).
Axolotls have been used as model organisms (i.e. organisms that are bred in labs and extensively used in scientific research) for over 150 years, and given their unique traits, it’s no surprise. They have been used to study an array of areas including neogenesis, development, and amphibian evolution (Reiß et al. 2015). So, I’ll ask again, are they worth the hype? I’d say so. Behind that unassuming face and wide set eyes lies a creature that has found a way to become the X-Men’s Wolverine of the animal kingdom, defying both age and permanent injury; qualities well worthy of our undying adoration.
Maden, M., Manwell, L. A., & Ormerod, B. K. (2013). Proliferation zones in the axolotl brain and regeneration of the telencephalon. Neural development, 8(1), 1.
Monaghan, J. R., Stier, A. C., Michonneau, F., Smith, M. D., Pasch, B., Maden, M., & Seifert, A.W. (2014). Experimentally induced metamorphosis in axolotls reduces regenerative rate and fidelity. Regeneration, 1(1), 2-14.
Prahlad, K. V., & DeLanney, L. E. (1965). A study of induced metamorphosis in the axolotl. Journal of Experimental Zoology, 160(1), 137-145.
Reiß, C., Olsson, L., & HOßFELD, U. W. E. (2015). The history of the oldest self -sustaining laboratory animal: 150 years of axolotl research. Journal of
Experimental Zoology Part B: Molecular and Developmental Evolution, 324(5),